The role of exciton lifetime for charge generation in organic solar cells at negligible energy-level offsets
The role of exciton lifetime for charge generation in organic solar cells at negligible energy-level offsets
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DOI：
10.1038/s41560-020-00684-7
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发表时间：
2020-08-31
期刊：
影响因子：
56.7
通讯作者：
Brabec, Christoph J.
中科院分区：
文献类型：
--
作者：
Classen, Andrej;Chochos, Christos L.;Brabec, Christoph J.
具有低能级偏移的供体-受体系统使有机太阳能电池具有高功率效率，但尚不清楚是什么驱动电荷产生。Classen等人表明，长激子寿命能够有效地分裂激子，从而产生自由电荷，同时还抑制电压损失。虽然一个非常小的能量水平偏移增加开路电压，目前还不清楚如何确切的电荷产生的影响。在这里，我们调查有机太阳能电池的混合物与最高占据分子轨道能级偏移（增量E-HOMO）之间的供体和受体，范围从0到300毫电子伏。我们证明，激子淬灭在一个可以忽略不计的增量E（HOMO）发生在接近原始材料的激子寿命的时间尺度上，这极大地限制了外量子效率。我们定量地描述了这一发现通过玻尔兹曼定态平衡之间的电荷转移状态和激子，并进一步揭示了一个长的激子寿命是决定性的，在一个可以忽略不计的增量E-HOMO保持有效的电荷产生。此外，玻尔兹曼平衡定量地描述了在一个非常小的增量E-HOMO的非辐射电压损耗的主要减少。最后，建议使用具有非常长激子寿命的高度发光的近红外发射体来实现高效的有机太阳能电池。
Donor-acceptor systems with low energy-level offset enable high power efficiency in organic solar cells yet it is unclear what drives charge generation. Classen et al. show that long exciton lifetimes enable efficient exciton splitting and thus generation of free charges while also suppressing voltage losses.Organic solar cells utilize an energy-level offset to generate free charge carriers. Although a very small energy-level offset increases the open-circuit voltage, it remains unclear how exactly charge generation is affected. Here we investigate organic solar cell blends with highest occupied molecular orbital energy-level offsets ( increment E-HOMO) between the donor and acceptor that range from 0 to 300 meV. We demonstrate that exciton quenching at a negligible increment E(HOMO)takes place on timescales that approach the exciton lifetime of the pristine materials, which drastically limits the external quantum efficiency. We quantitatively describe this finding via the Boltzmann stationary-state equilibrium between charge-transfer states and excitons and further reveal a long exciton lifetime to be decisive in maintaining an efficient charge generation at a negligible increment E-HOMO. Moreover, the Boltzmann equilibrium quantitatively describes the major reduction in non-radiative voltage losses at a very small increment E-HOMO. Ultimately, highly luminescent near-infrared emitters with very long exciton lifetimes are suggested to enable highly efficient organic solar cells.